The present invention relates to quadrature hybrid circuits, specifically a novel quadrature hybrid, and vector modulators, in general, and more specifically, a vector modulator contained in a small Chip Scale Package (CSP) using the novel quadrature hybrid circuit.
A quadrature hybrid is a type of circuit commonly used to split and combine signals. Quadrature hybrids are used extensively in power amplifiers and in other applications requiring noise reduction and signal clarity. They are also commonly used as couplers in applications requiring signals to be split or shared, such as sampling applications, testing applications and detector applications.
The quadrature hybrid is an effective tool for creating noise reduction circuits because it can be used to achieve both power division and phase shifting. One popular technique for eliminating unwanted noise signals, such as intermodulation distortion, is to divide the signal in half to form two separate signals that differ in by phase 180 degrees, and then recombine the two signals. This causes the two signals to cancel each other upon recombination; thus, eliminating the unwanted noise signal.
Prior to the development of quadrature hybrids, this process was accomplished by using power divider circuits to split signals, and the creating a phase shift by using transmission lines of different lengths calculated to result in the desired phase shift. However, creating a phase shift using transmission lines requires an extremely large section of valuable integrated circuit area, and thus is not practical for many applications.
In response to the need for a better means to divide and shift signals, the quadrature hybrid circuit was developed. The symbol used to represent a quadrature hybrid in a circuit diagram is shown in FIG. 1. The symbol as shown in FIG. 1 illustrates the function of the quadrature hybrid. An input is applied to the RF IN port 101. The input signal is divided between two outputs (102a and 102b). The two outputs are of equal amplitude, but vary in phase by 90 degrees. The remaining output of the quadrature hybrid 103 is run through a 50 ohm terminating resistor to ground. This port provides the required isolation to the quadrature hybrid. The actual circuit structure of a quadrature hybrid typically comprises at least a pair of inductors and a pair of capacitors. Various quadrature hybrid circuit structures are well known in the prior art.
Quadrature hybrids in accordance with the prior art have been constructed using discrete components. A pair of discrete inductors and a series of discrete capacitor are mounted on a printed circuit board to form the quadrature hybrid. This method is extremely space consuming and not practical for many applications. It also requires a large number of components and a lengthy assembly time.
In an effort to reduce the size and cost of the quadrature hybrid, efforts have been made to incorporate the elements into an integrated circuit. However, these efforts have met with only limited success. The inductors required can be incorporated into an integrated circuit; however, a additional capacitors are necessary for the quadrature hybrid to function properly. Adding discrete capacitors increases the space necessary to form the quadrature hybrid. The capacitors could be incorporated into the integrated circuit by thin film deposition techniques. However, this would substantially increase the necessary chip area for the hybrid circuit as capacitors require large areas of integrated circuit, typically more than an inductor.
One circuit that commonly employs quadrature hybrids for noise cancellation in power amplifiers is a vector modulator. Vector modulator circuits use a series of quadrature hybrids to provide a means to vary the phase and amplitude of RF signals. They are commonly used in power amplifiers to improve signal purity and provide digital modulation.
Power amplifiers use feed forward loops to improve signal purity. Intermodulation distortion of signals can occur from the mixing of multiple RF signals. These unwanted distortion signals are known as intermodulation products. To cancel these unwanted intermodulation products, a signal that is of equal amplitude and 180 degrees out of phase therewith is created and combined with each intermodulation product.
Until recently, this was accomplished using variable attenuators to control amplitude in series with variable phase shifters to control signal phase. This method, however, was a poor solution in high-volume production environments because of the numerous components and complex circuitry required. In addition, variable phase shifters contain an inherent phase change limitation. While it is possible to achieve a full 360 degree range of phase shifting using variable phase shifters, random access to specific phase values is not possible. This means that, to reach a specific phase value, the variable phase shifter must cycle sequentially through a range of values to settle on the desired phase shift. This causes the transient response time associated with this technique to be relatively long. This can cause a dramatic degradation in the feed forward amplification performance.
To overcome these concerns, vector modulators were developed. Vector modulators are used to vary both the phase and amplitude of RF signals. A vector modulator uses a series of quadrature hybrids to achieve phase and amplitude adjustment. FIG. 2 illustrates a vector modulator comprising an input quadrature hybrid 201 that contains two isolated outputs 202a, 202b. Each output feeds a separate voltage variable attenuator. Each voltage variable attenuator is comprised of a quadrature hybrid (203a and 203b) that is terminated on the two output ports with matched diodes (205a, 205b, 205c, 205d) or field effect transistors (FETs). This forms an absorptive attenuator. The isolated port of each voltage variable attenuator is fed to another quadrature hybrid circuit 207 acting as a power combiner. The output signal from the power combiner 207 is the RF input signal with a new phase and amplitude.
Vector modulators are incorporated into feed forward amplifiers to eliminate intermodulation distortion without the problems associated with the variable attenuators and variable phase shifters previously used.
While the use of vector modulators is a significant improvement over the circuits comprising variable attenuators in series with variable phase shifters, they still have several limitations. Vector modulators in accordance with the prior art are not suitable for high volume production techniques because they are comprised of several distinct elements as described above. A substantial amount of hand assembly is required to build the vector modulator, causing the devices to be costly. Because of the elements comprising the quadrature hybrids as discussed above, vector modulators are not capable of easy integration into an integrated circuit.
In addition, prior art vector modulators are relatively large devices as a result of the large size required by the hybrids contained therein. The minimum x and y dimensions of a typical integrated circuit vector modulator used in the prior art are approximately 25 mmxc3x9731 mm, making the vector modulator an area intensive component of the feed forward amplifier.
It is desired to develop a device that could perform the required vector modulation (phase shift and amplitude attenuation) but that is smaller and conducive to incorporation into an integrated circuit, and thus more suitable to high volume production techniques. This would enable reductions in cost and the amount of real estate required on the circuit board while preserving the performance advantages of the vector modulator.
The present invention provides an improved quadrature hybrid and an improved vector modulator incorporating the improved quadrature hybrid. The quadrature hybrid in accordance with the present invention removes the distinct capacitors that were necessary in the prior art and instead modifies the physical layout of the inductors of the quadrature hybrid to intrinsically provide the capacitance for the quadrature hybrid. By removing the capacitors, the quadrature hybrid in accordance with the present invention is suitable to fabrication into a small integrated circuit and capable of being used in chip scale applications.
The quadrature hybrid in accordance with the present invention provides the signal coupling and phase shifting that was accomplished in the prior art; however, the quadrature hybrid in accordance with the present invention does not require separate inductors and capacitors to achieve the desired results.
The quadrature hybrid in accordance with the present invention comprises two spiral inductors formed on an integrated circuit. A first spiral inductor and a second spiral inductor are formed on separate substrate layers within the intergrated circuit. The first and second inductor are parasitically coupled across the dielectric to allow an RF signal that enters the quadrature hybrid via an RF input to be divided between two outputs.
The spiral inductors are located such that the first spiral inductor and the second spiral inductor are formed on separate metal layers separated by an insulating layer. As a result, the two inductors in combination with the insulating layer act as two plates of a capacitor separated by a dielectric layer. By designing the spiral inductors to achieve the capacitance required for the quadrature hybrid, the need for adding additional capacitors is eliminated. The size and separation of the spiral inductors is controlled to allow the capacitance created to equal the capacitance required for the quadrature hybrid to function as desired.
An additional facet of the present invention provides an improved vector modulator that incorporates a series of quadrature hybrids in accordance with the present invention fabricated into an Monolithic Microwave Integrated Circuit (MMIC). The vector modulator in accordance with the present invention uses an MMIC chip in combination with a series of diodes to perform the vector modulation. All of the components are combined into a single, small chip scale package.
A vector modulator in accordance with the present invention can be packaged into a small chip scale package approximately 4 millimeters by 6 millimeters in size.
The chip scale package of the vector modulator in accordance with the present invention provides for superior electrical performance as a result of its unique configuration. By placing the quadrature hybrids of the vector modulator in a chip scale package with a paddle that provides an RF ground, the path lengths are reduced. In turn, this reduces the amount of inductance generated, thus improving the electrical performance of the vector modulator.